The term “shrink film” refers to a plastic wrapping film which has the characteristic of shrinking when it is heated to near the melting point of the film. These films are commonly manufactured from plastic resins such as polyvinyl chloride (PVC); polypropylene (PP); linear-low density polyethylene (LLDPE); low density polyethylene (LDPE); high density polyethylene (HDPE); copolymers of ethylene and vinyl acetate (EVA); copolymers of ethylene and vinyl alcohols (EVOH); ionomers (e.g. Surlyn™); copolymers of vinylidene chloride (e.g. PVDC, SARAN™ copolymers of ethylene acrylic acid (EAA); polyamides (PA); among others.
End uses of these films include food packaging (for example, oxygen and moisture barrier bag for frozen poultry, primal meat cuts and processed meat and cheese products for preservation of freshness and hygienics) and non-food packaging (for example, “overwraps” for protecting goods against damage, soiling, tampering and pilferage) during transportation, distribution, handling and display. An example of end use is found in retail sales where the films are wrapped air-tight around single or multiple items of compact disks, audio/video tapes, computer software boxes, magazines, confectionery, boxed products, single serve bowls, etc. Another example of end use is found in wholesale retailing where multiple containers of bottled and canned goods such as beverages, condiments and personal hygiene products are sold in bulk. Yet another example is found in courier shipping where single items of shrink-wrapped sporting goods and household appliances are now safely transported without the need for bulky protective cardboard cartons.
Collation shrink films are a particular type of shrink film. Collation shrink films are films that are wrapped around many packaging units (such as bottles or cans) and shrunk to keep the units within the package together. For example, collation shrink film may be wrapped around a multi-pack of drinks that are placed on a cardboard base and the film is then shrunk around the containers. The wrapping process typically involves a shrink oven or shrink tunnel in which the film is heated to cause the collation shrink wrapping to occur. The shrinking of the plastic film causes it to collapse around the multiple containers and hold them in place.
Collation films generally fall into two main categories, namely “transportation” (or shipping) films and “display” films.
Transportation film is typically used for bringing goods from the manufacturer to the retailer. The film is usually removed by the retailer and the products stacked on a shelf for sale. In the case of cans or bottles, a cardboard tray is usually placed below and the film acts as a tray overwrap. This type of shrink film provides cost savings in comparison to the use of cardboard boxes. The key attributes of transportation collation shrink films are toughness, good holding force and abuse resistance. The film appearance is of less concern and hence optical properties are less important in this application. “Display” collation film is generally used as secondary packaging in which the product is displayed for sale at the retailer and the customer takes home the product with the film. Examples include bottled water and beverages that is wrapped. Hence optical properties are more important in this sub-application.
Thus, the desired properties of collation shrink films (monolayer or multilayer) include 1) good shrinkage and holding force, 2) good stiffness, 3) good puncture resistance or toughness; 4) good optical properties and 5) good tensile strength at break. Some shrink films are made from “homogeneously branched” or “homogenous” resins.
Homogeneous resins may be prepared with so-called single site catalysts (e.g. metallocene catalysts) which are well known and widely described in the literature.
“Homogeneous” resin compositions have a uniform composition i.e. most of the polymer components of the composition have the same (or similar) comonomer content or density. These homogeneous resins typically exhibit excellent organoleptic properties and impact strength properties.
U.S. Pat. No. 6,306,969 describes a shrink film comprising a polymer composition characterized as having a density in the range of 0.88 gram/centimeter (g/cc) to 0.94 g/cc. The composition has two components which may be of different density but the density difference is small (from 0 to 0.018 g/cc).
U.S. Pat. Nos. 5,591,390 and 5,595,705 describe the preparation of shrink films made with a sub group of homogeneous resins, namely homogeneous resins having long chain branching. However, such resins are comparatively expensive.
More recently, another type of homogenous resin having long chain branching has found use in shrink film applications. This new resin, which is sold under the trade mark ENABLE by ExxonMobil Chemicals is believed to contain long chain branching and is recommended for use in the preparation of shrink films.